LED Battery Checkers

The `Battery Good` checker. When the button is pressed, the green LED will glow if the battery voltage is above the preset threshold. This version has a higher parts count than the `Battery Low` version, but a bonus is that it can drive an LED to be very bright, and the luminance is independent of the battery voltage. The simple expedient of a jumper wire on the back of the board made it unnecessary to hold the button down for this photograph. Besides, my finger is not very photogenic.
LED Battery Checkers - schematic

LED Battery Checkers - img1

According to Mr. Lung, the lowest safe voltage across a lead acid battery while in long term storage is 2.0 volts per cell, or 6.0 volts for a 6 volt battery. Gates Energy Products, the load must be disconnected from a lead acid cell when the voltage reads 1.8 volts or less, in order to avoid damage. For a 6 volt battery with three cells, this critical voltage is 3 x 1.8 volts = 5.4 volts. Choose your threshold depending upon your application. If there is a good chance that the battery will not be recharged soon after running down, be safe and go with the 2.0 volt per cell threshold. The TL431 has an internal threshold of 2.5 volts. Pot R2 is adjusted so that when the battery voltage equals the desired threshold, the wiper of R2 will be 1.25 volts. When the voltage across the battery is above the preset threshold, the voltage at the wiper of R2 is above 2.5 volts, the TL431 conducts, shunting current away from the LED, keeping the LED off. When voltage drops below the preset threshold, the voltage on the wiper of R2 drops below 2.5 volts, and the TL431 conducts less current, only up to about 1 milliamp, and the anode voltage rises until the LED conducts. I tested this with a 6 volt threshold rather than a 5.4 volt threshold. When adjusted for a 6 volt threshold, the LED turns on at 6.0 volts and then slowly fades out as the battery voltage decreases, until it reaches about 2.5 volts, when the LED is too dim to...

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